Signs of widely differing types and importance are generally used for controlling traffic, in order to assist the smooth handling of the traffic. This applies to marine navigation, to aircraft, for example to airports, as well as to all rail traffic, but in particular to road traffic.
Owing to the continuously rising amount of traffic, ever more traffic signs and illuminated signs are used for controlling traffic within cities and for controlling long-distance traffic. An increasing proportion of these signs are produced by light sources. Typical examples of this are the changing illuminated signs for light signaling systems at roadway crossings and the changing traffic signs at so-called freeway intersections.
At the moment, incandescent lamps are primarily used as the light sources for illuminated signs such as these. Incandescent lamps can fail by a short or discontinuity, thus resulting in a sign to be produced by them becoming distorted, with inadequate light intensity or not being displayed at all.
In order to prevent an incorrectly displayed illuminated sign from confusing those in the traffic, it must be switched off immediately in order to avoid a risk of accidents. In order to make it possible to check the availability of a safety-relevant sign, for example a red traffic light, a speed limit or a warning display, even when it is switched off, the monitoring must take place all the time. Thus, the monitoring must take place even when the corresponding sign is not active, that is to say when it is not actually illuminated.
Functional monitoring of the incandescent lamps that produce a sign can be carried out by passing a current through their filaments. The inertia of a filament refers to the fact that the serviceability of the incandescent lamp can be tested by passing a current through the filament for a short time, for example for 1 ms, without any light emerging.
The article “On-board multiplexing system checks car's lights automatically” on pages 68 and 70 of Electronics International may be cited as a reference for a cold lamp test such as this for automatically checking a car lighting system. In this case, commands to activate the lamps are passed via a microprocessor, the signals of the lamps and sensors are monitored, and the driver is informed of any malfunction via a display on the dashboard. A power transistor connects the lighting system to the car battery. A high-impedance voltage divider is connected in parallel with this, and its center potential is used as a criterion for checking a lamp state.
During operation, the microprocessor checks the value of the center potential of the voltage divider every 10 ms. When a light is switched on, the potential is 12 V and when it is off, the potential is 0 V. In the event of a short in the lamp circuit, the potential is, however, 0 V in both cases.
In order to obtain a positive indication for all possible faults and light operating modes, the test must be extended in order also to include the OFF mode when the light is switched on the ON mode when the light is switched off. A switched-on lamp is switched off for about 100 ms once every second by the system; when it is off, the system switches it on periodically for 100 ms every 40 seconds. The tests starts when the engine is started, and ends 100 s after the engine is stopped. This extended operation ensures that lamp failures are detected even during the period when the filament is cooling down.
Incandescent lamps are now increasingly being replaced by light-emitting diodes, which are also referred to in the following text for short as LEDs. This is being done since, as a low-maintenance and high-availability light source for optical signs, LEDs have many advantages for economic operation of light signaling systems.
One problem is that signs which can be produced by LEDs have until now been capable of being monitored only when they are in the switched-on state. It has therefore not been possible to use LED technology for safety-relevant signs, whose serviceability must also be monitored when they are switched off.
Owing to the effectively inertia-free conversion of current to light in LED light sources, a functional test analogous to the so-called cold lamp test was not feasible without production of disturbing, and thus unacceptable, light flashes. In complete darkness, when an LED is operated at its rated current, even pulses with a length of more than about 0.3 μs and with a continuous current of more than about 5 μA are noticeable in a disturbing manner, largely independently of the repetition rate. Signs with functional monitoring which produce sufficiently short and weak current pulses and can reliably monitor them have not until now been feasible at an acceptable complexity level.